We propose to continue micropuncture and clearance studies in rats utilizing electron probe analysis of tubular fluid designed to study: 1) the sites of altered calcium transport in the proximal and distal tubule of superficial nephron, and 2) the mechanisms of these alterations in terms of intratubular fluid compostition vs. extracellular fluid changes or changes in hormonal activity; to evaluate the role of deeper nephrons, microinjection and microperfusion techniques will be used.